1. Field of the Invention
The present invention relates to optical transmitters for use, for example, in a TPON (telecommunications over passive optical networks) network.
2. Description of the Prior Art
In a TPON network, a network head-end station (for example a telephone exchange) is linked to a plurality of remote terminations (for example street distribution points) by a passive optical splitting network (PON). There may be separate PONs for the downstream (head-end to terminations) and upstream (terminations to head-end) data directions.
In the TPON network, according to a bit transport system (BTS) proposed by British Telecommunications PLC, in the downstream direction data is broadcast by the head-end station to all the terminations in the network in a time division multiple access (TDMA) multiplexed frame (multiframe). In the upstream direction, the terminations transmit, one at a time, data pulses in predetermined time slots so that the data pulses reaching the head-end station from the different terminations are interleaved to form an upstream TDMA multiframe having a predetermined format.
Each remote termination includes an optical transmitter for producing optical signals in dependence upon the data pulses to be transmitted in the upstream TDMA multiframe to the head-end station. The optical transmitter includes an optical source, normally a semiconductor laser, to which electrical drive signals are applied to modulate the optical output power of the optical source. In view of unavoidable variations in optical output power from one optical source to the next for the same drive signal, and also of drift in the optical output power with age and temperature, the optical output power of the optical source is desirably controlled by a servo circuit. Such a servo circuit uses an optical sensor arranged, for example in the same package as the optical source, for monitoring the optical output power of the source so as to produce monitoring signals dependent upon the output power. The servo circuit applies drive signals to the optical source for controlling the optical output power thereof, in dependence upon the monitoring signals provided by the optical sensor, so as to tend to maintain that output power at a predetermined demand level during use of the transmitter.
When a transmitter is first connected to the network, or following an interruption in the transmitter power supply, the output power of the optical source is required to reach the predetermined demand level quickly and accurately. In a previously considered transmitter a so-called linear power setting operation has been performed for this purpose, in which the drive signals applied to the optical source at successive time instants are increased by a fixed adjustment amount and the resulting output power is monitored using the monitoring signals provided by the optical sensor. In such a linear power setting operation the drive signal accordingly increases linearly, from zero towards a predetermined maximum drive signal value, until it is determined that the optical output level, as measured by the optical sensor, has reached the predetermined demand level.
In the linear power setting operation the fixed adjustment amount, by which the drive signal changes from one time instant to the next, is set in dependence upon the desired accuracy of the power setting operation. For example, if it is required to set the optical output power to within 1% of the predetermined demand level, the adjustment amount must be 1% or less of the maximum drive signal value. Thus, for relatively high demand levels, the linear power setting operation requires up to 100 time instants to achieve 1% power setting accuracy.
In a data communications network such as a TPON network there are normally many terminations (for example up to 128) connected to the network. Linear power setting, as described above, is undesirable in such a data communications network. In this respect, in order that existing terminations can continue to operate uninterruptedly when a new termination is first connected to the network, a predetermined time slot is normally reserved periodically for network initialisation purposes. For example, in a TPON network using the BTS system mentioned above, an initial portion of each upstream TDMA frame is reserved for such purposes and, when first activated, a newly-connected termination must confine its optical transmissions to this initial portion to avoid upsetting operation of the existing terminations.
To achieve high efficiency in utilization of the TDMA multiframe for data transmission, the initial portion of the frame used for network control purposes should desirably be of a very short duration, so that in practice only one value of drive signal can be tried by a termination in each TDMA multiframe. As the duration of each TDMA frame is 10ms in the BTS system, it can take 2-3 seconds to set the optical output power of a newly-connected termination when linear power setting to a typical accuracy of 0.4% is performed.
When an entire TPON network is first initialized, power setting is required by all the terminations. Since only one termination can perform a power setting operation at any one time (because of the possibility of intersymbol interference if more than one termination transmits at a time) power setting by a network having 128 terminations can take up to 6 minutes. Thus, there is an unacceptably long delay before the network is available for use by all the terminations.